Chap. VII.] THEORY OF NATURAL SELECTION. 267 



we have to allow for the direct and definite action of 

 changed conditions of life, and for so-called spontaneous 

 variations, in which the nature of the conditions ap- 

 parently plays a quite subordinate part. Bud- variations, 

 such as the appearance of a moss-rose on a common rose, 

 or of a nectarine on a peach-tree, offer good instances of 

 spontaneous variations ; but even in these cases, if we 

 bear in mind the power of a minute drop of poison in 

 producing complex galls, we ought not to feel too sure 

 that the above variations are not the effect of some local 

 change in the nature of the sap, due to some change in 

 the conditions. There must be some efficient cause for 

 each slight individual difference, as well as for more 

 strongly marked variations which occasionally arise ; and 

 if the unknown cause were to act persistently, it is almost 

 certain that all the individuals of the species would be 

 similarly modified. 



In the earlier editions of this work I under-rated, as 

 it now seems probable, the frequency and importance of 

 modifications due to spontaneous variability. But it is 

 impossible to attribute to this cause the innumerable 

 structures which are so well adapted to the habits of life 

 of each species. I can no more believe in this, than 

 that the well-adapted form of a race-horse or greyhound, 

 which before the principle of selection by man was 

 well understood, excited so much surprise in the minds 

 of the older naturalists, can thus be explained. 



It may be worth while to illustrate some of the fore- 

 going remarks. With respect to the assumed inutility 

 of various parts and organs, it is hardly necessary to 

 observe that even in the higher and best-known animals 

 many structures exist, which are so highly developed 

 that no one doubts that they are of importance, yet their 



